As the title says i would like to know any critical pitfalls and disadvantages of series capacitors,on high power converters.

For example in a buck,if i require an input decoupling capacitor rated for at least 200v or an output LC filter capacitor:
Would it be better to get a big bulk caps,which are rated for 500v and above - usually these are expensive.
Or
Connect lower rated capacitors in series to meet the required voltage,I am concerned about the ESR and other losses in this case.?

Back in the day (yes, I'm old) a high value resistor was placed in parallel with each of the series capacitors. The effect was to attempt to prevent a capacitor having a high leakage resistance from seeing a higher voltage than those with lower leakage resistance. Folks with more modern know how may have a different opinion.

Without the resistors mentioned above, the applied DC will distribute itself according to the actual capacitance values. With low cost caps that have a +80%/-20% tolerance, it is very possible for your 500 V to be distributed 400V / 100 V instead of 250 / 250.

The issue here is leakage and resulting voltage distribution. You can get into a situation where the capacitor with the least leakage will take on more voltage than it can handle, leading to successive failure.

Generally this can be cured with balancing resistors but they defeat the purpose of the capacitor.

One way to limit the problem would be using tighter tolerance capacitors. But that would be a rather expensive solution.
Another option would be using lower value capacitors with much higher voltage rating each. But again, price would be an issue.

It's best to simply use a single capacitor with the proper voltage rating.

The right size in a single part is more economical and effective than a kludge made of extra resistors and a calculated risk based on capacitance tolerance and leakage currents (combined). And no, ceramic caps won't fix anything here that the right electrolytic cap won't fix.